Thyroid carcinoma is the most frequently occurring endocrine cancer. Most thyroid cancers are successfully treated with surgery and radioiodine (131I). However, poorly differentiated thyroid cancer (PDTC) loses its ability to take up radioiodine leading to disease recurrence and death. Additionally, patients with anaplastic thyroid cancer (ATC) show very poor prognosis and survive for less than 6 months. These findings hold true for veterans as well; moreover, veterans could be at a higher risk for developing thyroid cancer due to their previous exposure to radiation and the enhanced risk of potential exposure to radiation amongst individuals who serve in the US armed forces. Therefore, a treatment that can selectively kill tumor cells that are unresponsive to radioiodine treatment and the anaplastic thyroid cancer cells is highly desired. TRAIL is one such potential therapeutic agent. TRAIL is a cytolytic drug that can selectively kill cancer cells while sparing most normal cells. However, its clinical utility in cancer treatment has been limited due to the expression of TRAIL-resistance factors. Map kinase Activating Death Domain containing protein (MADD; a.k.a. IG20) can confer resistance to TRAIL- induced apoptosis in a variety of thyroid and other cancer cells. While knockdown of MADD in cancer cells rendered them more susceptible to apoptosis, it had no apparent effect on normal cells, suggesting that it might serve as an ideal target for developing cancer therapy. The ability of MADD to act as a TRAIL resistance factor is dependent upon it's phosphorylation by Akt (i.e. pMADD). Interestingly, in TRAIL sensitive cancer cells, TRAIL treatment reduced the levels of pAkt and pMADD, while they remained unaltered in TRAIL- resistant cells. However, reducing the pMADD levels by suppressing the Akt activity restored TRAIL sensitivity to TRAIL resistant cells. This is profoundly interesting because the most aggressive forms of thyroid cancers, such as PDTC and ATC, show constitutive activation of Akt due to activating mutations in Ras or Akt1, PIK3CA gene amplification or inactivating mutations in PTEN, which can enhance tumor survival and growth. The PI3K/Akt signalling pathways are attractive therapeutic targets and drugs that can suppress this signaling pathway are in clinical trials. However, most of these are cytostatic and not cytolytic and thus allow tumors to persist, develop resistance and metastasize. Thus there is need for developing newer modalities of effective treatment. A clearer understanding of the role of pMADD, in regulating apoptosis of ATC cells could enable us to develop effective methods to kill them. We hypothesize that TRAIL can be used to effectively kill most anaplastic thyroid cancer cells. However, in TRAIL resistant anaplastic thyroid cancer cells, MADD function can be repressed to sensitize them to TRAIL-induced apoptosis. To test this hypothesis in Aim-1 we will determine if Akt phosphorylation of MADD contributes to TRAIL-resistance in anaplastic thyroid cancer cells; in Aim-2 we will test the in vivo therapeutic utility of down modulating MADD function in anaplastic thyroid cancer using an orthotopic xenograft athymic nude mouse model; and in Aim-3 we will determine the levels of MADD and pMADD in anaplastic thyroid cancers and see if they correlate with TRAIL-resistance of primary cells derived from those cancers. Apoptosis induced by TRAIL as well as by MADD down modulation is primarily seen in cancer cells and not normal cells. Therefore, the proposed novel studies on down modulating MADD function in anaplastic thyroid cancer to enhance TRAIL sensitivity is of high clinical significance and highly relevant to VA mission.